1. Field of the Invention
The present invention relates to a recording and reproducing apparatus for performing the recording process by forming microscopic pits and projections on a substrate surface with a recording voltage applied to a probe brought in proximity to the substrate surface, and for performing the reproduction process by detecting a current that flows according to the pits and projections through the probe brought in proximity to the substrate surface. The invention also relates to a high-density recording medium for the recording and reproducing apparatus.
2. Description of the Prior Art
Conventionally, the so-called STM (Scanning Tunnel Microscope), which detects the pits and projections of the surface by detecting a tunnel current developed when a microscopic probe is brought in close proximity to the substrate surface, has been used for fine evaluation of surface shape. In more recent years, further attempts have been made to implement the recording by forming pits and projections of the atomic order on the substrate surface by using the STM.
Generally, a conventional STM has the following arrangement. As shown in FIG. 5, piezoelectric elements 27, 28, 29 are arranged in a cylindrical shape, and a chip 31 is located at an end of an actuator 30 that will be displaced axially by drive voltages applied to those piezoelectric elements. The pits and projections of the surface are measured by detecting a tunnel current that will flow when the chip is brought in proximity to the surface of the recording medium.
In this arrangement, a large current, when flowed through the chip, allows microscopic crater-like pits to be made in the surface of the recording medium, whereby the recording can be accomplished. The recording can be done also by making use of variations in the magnetized state or dielectric constant of the recording medium or the like, not only by making crater-like pits. Detailed description of these aspects can be found in literatures (e.g., NIKKEI MICRODEVICES, July 1991, p. 89).
Japanese Patent Laid-open Publication No. 4-206536 describes an attempt to provide a recording apparatus using the principle of the STM. In this example, a structure having a plurality of chips arranged one by one is mounted on a piezoelectric element, and the chips are driven in a parallel fashion so that signal recording and reproduction is attained.
Japanese Patent Laid-Open Publication No. 4-157644 describes a method in which the STM chip is mounted on a floating type slider used for the recording on magnetic hard disks, whereby recording is enabled. Further, in Japanese Patent Laid-open Publication No. 6-176410, the present inventors have proposed a method of forming a slider and a cantilever integrally.
Currently, commonly known are two methods of detecting STM signals. One method is to do scanning in such a way that the STM signal current is maintained constant. More specifically, once an STM current is detected, the height of the chip is controlled by an actuator so that the current is kept constant. In doing this, pits-and-projections information of the recording medium surface is obtained from the drive voltage of the actuator. This method is widely used by virtue of its permitting a wide dynamic range of measurement.
The other method is to do scanning with the height of the chip maintained constant. That is, scanning is performed while the spacing between the chip and the recording medium surface is maintained constant. Then, pits-and-projections information of the surface is obtained from the magnitude of the STM current that results from the scanning.
The above two methods of detecting STM signals, when applied to a recording and reproducing apparatus using the STM, would give rise to the following problems.
Firstly, the two detection methods are both considerably weak to external vibrations, to a disadvantage. This is because, in either case of the methods, any vibration of the chip or the recording medium due to external vibration would be detected as pits-and-projections information as it is.
In the detection method in which the STM current is maintained constant, when the chip or recording medium vibrates due to external vibration, the piezoelectric element is driven in such a direction as to correct the vibration. The drive voltage of the actuator is recognized as pits-and-projections information and therefore erroneous pits-and-projections information is detected.
The case is the same also with the detection method in which the chip height is maintained constant. That is, if the chip vibrates with external vibration applied thereto, variations in the STM current due to this are detected directly as pits and projections. As a result, erroneous pits-and-projections information is detected also in this case.
Accordingly, there has hitherto been a need of installing the whole recording and reproducing apparatus on a vibration isolator so that external disturbance can be shielded to a maximum for prevention the possibilities of the above-mentioned misdetection. This need would inevitably cause the recording and reproducing apparatus to be a large-size, heavyweight, and expensive one as a whole.
As another problem, when recording is implemented by the detection method in which the STM current is maintained constant, the recording signal would give external disturbance to signals for use of driving the piezoelectric element. In more detail, for recording pits and projections, a large recording voltage is applied to the chip to make a flow of current. The current flowing in this process is greater than normal STM currents. Besides, since this current flows intermittently according to the pattern of the signal of pits and projections, a steady, weak STM current for maintaining constant a spacing between probe and sample cannot not be detected while the recording is being performed. Therefore, it is difficult to maintain constant the spacing between chip and recording medium.
For the reproducing process, in areas in which pits and projections are recorded, there arise variations in the STM current due to the pits and projections. In the case of the method in which the STM current is maintained constant, it is necessary to drive the chip vertically after the pits and projections that cause the variations in the STM current. However, if a signal is to be recorded at a very high density and reproduced at high speed, the frequency of the reproducing signal would reach the order of a few MHz to a few GHz so that the chip could hardly be made to track this frequency.
In the case of the method in which the chip height is constant, it is only required to detect variations in the STM current, in which case high speed reproduction is relatively easy to accomplish. However, in this case, the STM current could not be detected unless the chip height is within a specific range of, for example, 10 nm or less. Therefore, there is a limitation to such areas that the spacing between chip and recording medium falls within a specific range, disadvantageously.